1. Field of the Invention
This invention relates to a device for testing or evaluating an amplifier which amplifies frequency components of n waves, n being an integer of 2 or greater, at the same time by supplying the frequency components to the amplifier.
2. Description of the Related Art
For example, a method of simultaneously amplifying a plurality of carriers such as eight, 16, or 24 waves is used in a base station system for mobile communication. FIG. 7 shows a schematic block diagram of a system using such a method.
The system shown in FIG. 7 comprises oscillators 10-1, 10-2 . . . , 10-n, a combiner 12, an amplifier 14, and a communication antenna 16. The oscillators 10-1, 10-2, . . . , 10-n oscillate at predetermined frequencies f.sub.1, f.sub.2, . . . , f.sub.n respectively. The combiner 12 combines oscillation outputs of the oscillators 10-1, 10-2, . . . , 10-n and feeds the combined result into the amplifier 14, which then amplifies it. The amplified result is transmitted through the communication antenna 16. In the system shown in FIG. 7, a plurality of carriers having frequencies f.sub.1, f.sub.2, . . . , f.sub.n are combined and then amplified.
Performance evaluation of such a system, particularly of the amplifier 14 requires a testing device which has a signal source for generating a large number of carriers at the same time. If a testing device appropriate for such an application is designed based on the practical sense of those skilled in the art, the schematic configuration of the testing device will be presented as shown in FIG. 8.
The testing device 18 shown in the figure comprises a reference oscillator 20, oscillators 22-1, 22-2, . . . , 22-n, and a combiner 24. Each of the oscillators 22-1, 22-2, . . . , 22-n is configured as a PLL (phase locked loop) circuit, for example. The reference oscillator 20 oscillates at a predetermined reference frequency and feeds its oscillation output into the oscillators 22-1, 22-2, . . . , 22-n. The oscillators 22-1, 22-2, . . . , 22-n use the oscillation output of the reference oscillator 20 as frequency and phase reference and oscillate at their respective predetermined frequencies f.sub.1, f.sub.2, . . . , f.sub.n. Each division ratio of the reference frequency at the oscillators 22-1, 22-2, . . . , 22-n is set to a predetermined value by a controller (not shown). The combiner 24 combines oscillation outputs of the oscillators 22-1, 22-2, . . . , 22-n and supplies the result to the amplifier 14 to be tested.
If the testing device 18 is thus configured as a PLL synthesizer, only one reference oscillator 20 is sufficient to give the reference frequency, simplifying the configuration. On the other hand, since the reference of the oscillation frequencies f.sub.1, f.sub.2, . . . , f.sub.n and oscillation phases of the oscillators 22-1, 22-2, . . . , 22-n is given by a single reference oscillator 20, the phase correlation among the oscillators 22-1, 22-2, . . . , 22-n becomes strong.
Therefore, a state that peaks in oscillation output waves of the oscillators 22-1, 22-2, . . . , 22-n are prone to overlap each other, and a state that any two among output waves of the oscillators 22-1, 22-2, . . . , 22-n are prone to have opposite phase to each other, periodically occur. Resultantly, the peak power of signals given to the amplifier 14 to be tested varies remarkably and periodically.
For example, as shown in FIG. 9, if the peaks in oscillation outputs of the oscillators 22-1, 22-2, . . . , 22-n overlap each other at the timing indicated by a broken line, the peak power of the signal output from the combiner 24 becomes large. In contrast, if the phases of the oscillation outputs of two oscillators, for example, 22-1 and 22-2 become opposite as shown in FIG. 10, the peak power of the signal output from the combiner 24 becomes small. Note that the average power of the output signals of the combiner 24 does not vary in any states described above.
On the other hand, characteristics such as attenuation of intermodulation distortion of amplification elements making up the amplifier 14 to be tested, for example, transistors, depend largely on the peak power of the input signals. Therefore, if the peak power of the signal source used with the testing device (in FIG. 8, peak power of output of the combiner 24) varies greatly, it becomes difficult to evaluate the performance of the amplifier 14 in a true-to-fact and precise manner.